1. Field of the Invention
The present invention relates to a battery management system. More particularly, the present invention relates to a method for correcting a state of charge (SOC) of a battery in a vehicle using electrical energy, and a battery management system using the same.
2. Description of the Related Art
Vehicles with an internal combustion engine using gasoline or diesel have caused serious air pollution. Accordingly, various undertakings for developing electric or hybrid vehicles have recently been made to reduce such air pollution.
An electric vehicle is run by using electrical energy output by a battery. Since the electric vehicle mainly uses a battery formed of one battery pack including a plurality of rechargeable/dischargeable secondary cells, it produces no emission gases and generates less noise.
A hybrid vehicle is a gasoline-electric hybrid vehicle that uses gasoline to power an internal-combustion engine and a battery to power an electric motor. Recently, hybrid vehicles using an internal-combustion engine and fuel cells and hybrid vehicles using a battery and fuel cells have been developed. The fuel cells directly obtain electrical energy by generating a chemical reaction while hydrogen and oxygen are continuously provided.
Such a vehicle using electrical energy drives a generator with residual power to charge the battery when an engine outputs high power, and drives a motor by using the electric power of the battery to overcome insufficiency of power when the engine outputs low power. In this case, the battery is discharged.
Since battery performance directly affects a vehicle using electrical energy, it is generally required that each battery cell has great performance. Also, it is generally required to provide a battery management system (BMS) for measuring voltage and current of the overall battery to efficiently manage charging/discharging operations of each battery cell.
Thus, a conventional BMS measures values of a voltage, a current, and a temperature of the battery to estimate a state of charge (SOC) through an operation, and controls the SOC to improve fuel consumption efficiency of the vehicle.
The SOC is controlled to provide a balance between motor driving for power assist during acceleration and energy recovery (e.g., regenerative braking) during deceleration. In general, for example, the battery over-discharging is controlled when the SOC is decreased to 50% and the battery over-charging is controlled when the SOC is increased to 70%, to maintain the range of the SOC of the battery within 50% to 70% to thereby keep the SOC close to the center of the control.
In order to accurately control the SOC, it is essential to accurately estimate SOC of the battery in the charging/discharging state.
Conventionally, there are two SOC estimation methods. One is to measure a charge current and a discharge current (charge current has a negative (−) sign and discharge current has a positive (+) sign), multiply the current value with charge efficiency, integrate the multiplication results for a predetermined time period to calculate integration capacity, and estimate an SOC based on the integration capacity. The other method is to measure and memorize a plurality of pair data of a discharge/charge current and a corresponding terminal voltage of a rechargeable battery, obtain a one-dimensional approximation line (voltage V-current I) from the pair data, and estimate an SOC based on the no-load voltage (open circuit voltage, OCV) that is a voltage (V section of V-I approximate line) calculated in correspondence to a current value of zero.
However, in the case of the SOC estimation method that uses the integration capacity, the charge efficiency applied to the integration of the current value depends on the SOC value, the current value, and the temperature, and therefore it is difficult to estimate the most adequate charge efficiency for each condition. Moreover, it is also difficult to calculate the amount of self discharge when the battery is in a non-use state.
Therefore, the conventional SOC measuring method that uses the integration capacity cannot measure an accurate SOC because an error between an exact value and an estimated value of the SOC increases as time elapses.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.